In the electronics industry, printed wiring boards are widely used. Substrates and component leads on the boards are coated and/or joined by solder in rapid, automated mass production processes. Scrutiny of every joint is not feasible so that highest levels of reliability and quality and the avoidance of defects are sought.
To meet the continuing drive for greater compactness, the proximity of substrates and joints on boards has increased. This has resulted in increasing propensity for the formation of undesired connections or bridges between substrates and joints. Hence the electronics industry has avidly sought processes that reduce the occurrence of this particular defect, among others, in the prevailing method of mass production soldering, namely wave soldering.
The next most prevalent method of soldering, reflow soldering, in addition to bridging, experiences other defects peculiar to it, namely, dewetting of substrates by solder, discoloration of boards and white haze on boards. Dewetting of substrates by the solder results in discontinuous coating by solder of substrates, and reduced adherence of solder to substrates, and consequently, joints of low strength and reliability. Discoloration on circuit boards after the reflow process raises concerns that the board has been damaged in some manner. Since the manner of damage is often not apparent, discoloration is tolerated to a large extent. White haze occurring on boards during the soldering process is a form of discoloration of particular concern. Many coatings used to protect soldered circuit boards do not adhere well to the white haze areas on the board.
An improved wave soldering process directed to reduced bridging defects and dross generation and an improved reflow soldering process directed to reduced dewetting of substrates by solder, white haze on boards and discoloration on boards was disclosed in U.S. Pat. No. 5,071,058, dated Dec. 10, 1991, issued to Nowotarski, titled "Process For Joining/Coating Using An Atmosphere Having A Controlled Oxidation Capability." In the disclosed processes, the soldering was conducted in a controlled oxidizing atmosphere having an oxygen concentration of from about 0.001% to about 10% by volume. Conventional fluxes and solders were used, and no additives were disclosed.
While the processes disclosed in the cited patent provided reductions in the defects named, including bridging, the instant invention provides still lower levels of such defects.